Coherent pulse radar system

Abstract

A coherent, frequency agile pulse radar system in which each transmitted pulse is made up of subpulse pairs having frequencies of omegao±DELTAi forces a jammer to broadband jam the system in order to ensure jamming the actual operating frequency of the system. On reception, a separate receiver channel is utilized for the detection of each subpulse frequency. Within the processing system, the signal return for a subpulse having a frequency of omegao+DELTAl is multipled times the signal return for the subpulse having a frequency of omegao-DELTAl to provide a combined signal which is free of the offset frequencies. This combined signal is a standard coherent frequency agile pulse doppler radar signal with the exception that the carrier frequency and the doppler frequency are both doubled. Moving target indicating or doppler processing may be used. N-time-around range ambiguities can be eliminated and sensitivity time control can be used to remove close-in clutter without limiting the pulse repetition rate.

What is claimed is:  
     
       1. In a coherent pulse radar system of the type including means for providing and transmitting a frequency coded transmission pulse and means for receiving and processing return signals coherently in accordance with said transmitted frequency code, the improvement wherein; 
       said means for providing comprises means for generating a transmission pulse comprised of N pairs of sub-pulses, N an integer greater than or equal to one, with the first subpulse of the i th  pair of subpulses having a sub pulse frequency of ω o +Δ i  and the second subpulse of said i th  pair of subpulses having a subpulse frequency of ω o −Δ i , where ω o  is the carrier frequency, Δ i  is a frequency offset and i is an integer, l≦i≦N, and  
       said means for receiving and processing comprises:  
       a separate channel responsive to each subpulse frequency in said transmission pulse for converting the return signal at that sub-pulse frequency to a baseband signal, and  
       means for multiplying said baseband signal for said first subpulse of each pair by said baseband signal for said second subpulse of said pair to yield a combined signal for that pair which is free of said frequency offset.  
     
     
       2. The improvement recited in claim  1  wherein: 
       said means for generating comprises means for providing each of said subpulse pairs in said transmission pulse with a different Δ i  value, whereby 2N different frequencies are present in said transmission pulse.  
     
     
       3. The improvement recited in claim  1  wherein: 
       said means for generating comprises means for placing said two subpulses of a pair adjacent in time in said transmission pulse.  
     
     
       4. The improvement recited in claim  1  wherein: 
       said means for generating comprises means for spacing said two subpulses of a pair in time within said transmission pulse by at least one other subpulse.  
     
     
       5. The improvement recited in claim  1  wherein: 
       said means for generating comprises means for placing said two subpulses of a pair in an overlapping in-time relationship within said transmission pulse.  
     
     
       6. The improvement recited in claim  1  wherein said means for generating comprises means for changing the carrier frequency ω o  from one transmission pulse to the next. 
     
     
       7. The improvement recited in claim  1  wherein said means for generating comprises means for changing the frequency offsets Δ i  from one transmission pulse to the next. 
     
     
       8. The improvement recited in claim  1  wherein said means for generating comprises means for providing a coherent local oscillator signal for each of said subpulse frequencies. 
     
     
       9. The improvement recited in claim  1  wherein said means for receiving and processing comprises means for doppler processing said combined signal. 
     
     
       10. The improvement recited in claim  1  wherein each of said separate channels includes means for applying sensitivity time control to said return signal at its subpulse frequency in times relation to the transmission of said sub-pulse frequency. 
     
     
       11. In a method of processing radar signals including the steps of generating and transmitting a frequency coded transmission pulse, receiving a return signal, coherently detecting said return signal in accordance with said transmitted code, and processing the resulting baseband signals to determine the presence of target returns in the return signal, the improvement wherein: 
       said generating step comprises generating a frequency code comprising N subpulse pairs, N an integer l≦N, with one subpulse of the i th  pair having a frequency of ω o +Δ i  and the other subpulse of said i th  pair having a frequency of ω o −Δ i , where ω o  is a carrier frequency, Δ i  is a frequency offset and i is an integer l≦i≦N; and  
       the following step is performed after said coherent detecting step and before said processing step:  
       multiplying the baseband signal detected from said ω o +Δ i  subpulse times the baseband signal detected from said ω o −Δ i  subpulse to produce a combined baseband signal free of said frequency offsets.  
     
     
       12. The improved method recited in claim  11  further comprising the step of: 
       bringing said ω o +Δ i  baseband signal into time synchronism with said ω o −Δ i  baseband signal prior to performing said multiplying step.  
     
     
       13. The improved method recited in claim  11  wherein said processing step comprises doppler processing said combined signal. 
     
     
       14. The improved method recited in claim  11  wherein: 
       said generating step comprises generating said ω o +Δ i  and said ω o −Δ i  subpulses sequentially in time.  
     
     
       15. The improved method recited in claim  11  wherein: 
       said generating step comprises generating said ω o +Δ i  and said ω o −Δ i  subpulses non-sequentially in time with at least one subpulse having a different frequency offset disposed between them in time.  
     
     
       16. The improved method recited in claim  11 , wherein said generating step comprises generating said ω o  +Δ i  and said ω o  −Δ i  subpulses simultaneously.